A method of casting under pressure is known in which the pressure is produced by a piston immersed in the melt together with the cylinder, which is filled with melt through a hole opened and closed by the piston during its idle and working stroke, respectively. This method is known as pressure casting with a hot chamber.
It is a drawback of this method that the piston and the cylinder are subjected to the strong erosive and corrosive action of the melt; for this reason it is not possible to produce high pressures inside the mold, particularly at the end of the crystallization process of the casting, since between the piston and the cylinder there is always a large gap. Moreover, the use of this method is limited only to alloys of low aggressiveness with respect to iron, such as magnesium and, to a certain degree, zinc alloys.
In another known method the pressure acting over the melt is produced by pressurized gas. In this method the filling of the mold by the melt is effected as a result of a differential between the pressures in the furnace space and inside the casting mold.
A substantial drawback of this method is that, for increasing the pressure at the end of the process of filling the mold with melt, it is necessary to deliver fresh quantities of highly pressurized gas. This increase of the pressure is effected comparatively slowly and cannot be utilized for both shape formation and structure formation of the casting. Another drawback that the process uses large quantities of pressurized gas introduced from the outside, and this results in high material and energy losses. The large gas masses often worsen the conditions of mass exchange between the gas phase and the melt.
In pressurized-gas casting apparatus a vessel operating under internal pressure receives a furnace with the crucible containing the melt. By means of a runner tube, the casting mold is connected to the melt and when pressurized gas is introduced inside the pressure vessel, the melt fills the casting mold
A drawback of such an apparatus is the large volume which must be filled with pressurized gas, and the impossibility of increasing sharply the pressure at the final stage of filling the casting mold with melt and of maintaining this pressure during the solidification of the casting. Another drawback is the large consumption of gas since, for the removal of the casting from the mold it is necessary to release the pressure and then, for the subsequent casting, to introduce again pressurized gas with material and energy losses.